Alkylation is the introduction of an alkyl group into a molecule. An alkylation process is commonly used in refinery operations for the production of highly branched C.sub.7.sup.+ to C.sub.9.sup.+ paraffins, e.g., the production of high octane isooctane from isobutane and isobutene. Paraffin alkylation can be conducted thermally or catalytically. Typically, the catalytic alkylation of isoparaffins with olefins is conducted in the presence of a sulfuric acid or hydrogen fluoride catalyst. In such processes the gaseous reactants are fixed as liquid products suitable for incorporation within motor fuels to boost the octane.
Catalytic reforming (hydroforming) is a well established industrial process employed by the petroleum industry for improving the octane quality of naphthas or straight run gasolines. In reforming, a multi-functional catalyst is employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Noble metal catalysts, notably platinum, or metal promoted platinum catalysts, are currently employed, reforming being defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst.
Reforming reactions are both endothermic and exothermic, the former predominating, particularly in the early stages of reforming with the latter predominating in the latter stages of reforming. In view thereof, it has become the practice to employ a plurality of adiabatic fixed-bed reactors in series with provision for interstage heating of the feed to each of the several reactors. There are two major types of reforming. In semi-regenerative reforming, the entire unit is operated by gradually and progressively increasing the temperature to compensate for deactivation of the catalyst caused by the coke deposition, until finally the entire unit is shut down for regeneration, and reactivation, of the catalyst. In cyclic reforming, the reactors are individually isolated, or in effect swung out of line by various piping arrangements, the catalyst is regenerated to remove the coke deposits, and then reactivated while the other reactors of the series remain on stream. A "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, and is then put back in series. In either type of reforming, hydrogen is produced in net yield, the product being separated into a C.sub.5.sup.+ liquid product, e.g., a 160.degree. F./430.degree. F. or C.sub.5.sup.+ /430.degree. F. product, and a hydrogen rich gas a portion of which is recycled to the several reactors of the process unit.
In this country, most high octane gasoline is produced from alkylation and catalytic cracking. Reforming, at least in a relative sense, is used to a somewhat lesser extent to produce high octane gasoline. In some other parts of the world, there is often low cracking and alkylation capacity and large reforming capacity. Reforming is often practiced abroad more widely than any other operation except, of course, crude distillation. In any regard, internally there is often excess reforming capacity. Moreover, there is often a need for the production of isobutane, a useful alkylate feed.
Reformer feeds do not normally contain any butane, either n-butane or isobutane (the desirable isomer for alkylation purposes), although both are produced during reforming, and found in the reformate. The ratio of the n-butane and isobutane found in the reformate, however, is limited by equilibrium conditions. Whereas some of the n-butane produced in the reformate can be recycled, or n-butane added to the reformer feed to increase isobutane production, the production of isobutane by this method is severely inhibited due to the presence of substantial concentrations of isobutane in the recycle gas.
Nonetheless, it is the prime objective of this invention to provide a new and improved process for the simultaneous conversion of n-butane, including added amounts of n-butane, to isobutane, and the recovery of isobutane from a reforming unit.
In particular, it is an object to provide a new and improved reforming process for the increased isomerization of butane to produce isobutane, and the coproduction of isobutane and a C.sub.5.sup.+ liquid reformate.
These objects and others are achieved in accordance with the present invention which comprises a new and improved mode of operating a reforming unit to co-produce isobutane, and an essentially C.sub.5.sup.+ liquid product. A naphtha feed which contains n-butane, or naphtha and n-butane as separate streams, is fed into the land reactor of a multiple reactor reformer unit, with hydrogen, and reacted at reforming conditions over a reforming catalyst, in generally conventional manner. Suitably, however, from about 2 percent to about 30 percent, preferably from about 5 percent to about 20 percent, of the feed to the reformer is constituted of n-butane, based on the total volume of feed to the reforming unit. The product from the reforming unit contains n-butane and isobutane in admixture, C.sub.5.sup.+ liquid, and lighter hydrocarbons, inclusive of hydrogen. The reformate from the last reactor of the series is cooled and separated into vapor and liquid, the vapor fraction is passed into an absorber, the liquid fraction is passed to a stabilizer, and a portion of the stabilized reformate from the stabilizer is countercurrently contacted within the absorber as a lean oil with said vapor fraction to strip primarily isobutane and heavier components from the vapor. The isobutane-denuded vapor fraction is separated into two portions, a first portion which is sent to other refining units, and a second portion which is recycled to the reforming unit as feed. The isobutane-containing fat oil from the absorber is sent to the stabilizer from which can be taken an isobutane rich stream and a conventional C.sub.5.sup.+ liquid reformate as separate products. The isobutane rich stream can be subsequently processed to recover isobutane for use as alkylation feedback.
In accordance with this process, an absorber is included in the downstream processing facilities. An admixture of isobutane and heavier components from the reforming unit are separated in the absorber via countercurrent extraction with a portion of the stabilized reformate. Both isobutane and n-butane are recovered from the stabilizer overhead product, the isomers separated, and the isobutane recovered. Isobutane is then sent to an alkylation unit, and all or a part of the n-butane is recycled to the reforming unit for conversion into an equilibrium admixture of n-butane and isobutane. Absorption of the separator overhead may also be carried out using any other type of lean oil such as extraneous heavy naphtha in the subsequent butane recovery. Thus, n-butane, which can be added with the naphtha feed, is in net-effect isomerized to isobutane providing an isobutane rich stream from which isobutane can be recovered for use as an alkylated feed. In addition, energy credits in the form of a higher purity, lower molecular weight recycle gas are provided. Moreover, placing the absorber upstream of the recycle gas compressor provides furnace fuel savings, and reduced compressor horsepower requirements. The higher purity recycle gas permits a lower recycle rate.
These features and others will be better understood by reference to the following more detailed description of the invention, and to the drawings to which reference is made.